We use the schematic to drive the PCB. The PBC BoM is used as a reference for a SAP BoM. SAP is the final say on the BoM and the schematic and PCB must agree with it. SAP is were all the non-electrical parts are added to the BoM and they are not on the schematic or the PCB file.
Allen Maddox
________________________________
From: "Brooks, William" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, June 26, 2013 2:10 PM
Subject: [DC] Questioning the trend to use the schematic diagram as a Bill of material generation source
I want to examine and get input on is the trend to use the schematic diagram as a Bill Of Material or BOM/Parts List generation sole source...
This subject is on my mind more and more and I think it needs an in depth discussion.
What I see happening is engineers who have not had formal drafting training using the schematic as a vehicle to automatically create a complete BOM for a PWA or PCA depending on where and how you term the printed circuit assembly with all the components installed.
The traditional documentation guide book for drafting of electronic systems has its roots in the Department of Defense standards... which were adopted and published by Global Engineering Documents in a Drawing Requirements Manual which is now in its 11th edition authored by JEROME H. LIEBLICH. If you want to review it, you can find it at http://www.global.ihs.com/
On a separate evolution line is the CAD or EDA companies that create the tools that have been used since the computer was starting to become a graphics engine... we can thank them for the creation of drafting tools that do more than just draft a drawing... without them we would not have our boards done in the time constraints we enjoy today...
My experience predates the computer so I remember using a drafting table and light table to create my boards...
According to historical standards, the controlling document for a Printed circuit assembly is the Bill of Material or Parts List and an Assembly drawing...
All revision control stems from this document that lists the electrical and mechanical parts and processes including the bare board that make up the PCA. The Assembly drawing is the controlling document by which the part is assembled and inspected.
Traditionally the Assembly Drawing was considered the first sheet of a parts list... often if the list was small it would be documented right on the Assembly drawing... but since more often there were so many parts to list we usually made a separate Parts List (Bill of Materials) but it kept the same part number as the BOM and ASSEMBLY drawing and the revision letters typically tracked... though provision was made in the DOD specs to handle the revs separately too, as long as it was referenced on the BOM. So the BOM was the master top level controlling document for building a PCA/PWA.
The way I am seeing the CAD tools being used nowadays is to drive the BOM from the schematic diagram... and because of this many designers and engineers are creating symbols for non-electrical or mechanical parts on the schematic to get the BOM to generate a complete bill of materials for the PCA.
Should we be doing this... ???
All of the implications may not be so obvious to everyone... I would like to get other professional's opinions and observations regarding this practice ...
Best regards,
William Brooks, CID+
Senior MTS (Contract)
2747 Loker Ave West
Carlsbad, CA 92010-6603
760-930-7212
Fax: 760.918.8332
Mobile: 760.216.0170
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